This invention relates to microwave transistor testing using automatic microwave impedance tuners in order to synthesize fundamental and harmonic reflection factors (or impedances) at the input and output of said transistors.
Impedance tuners at RF and microwave frequencies are based, in most cases, on the slide-screw principle (FIG. 1). Such tuners comprise a low loss parallel plate transmission line (slabline, (1)) and metallic probes (slugs, (3)), which are inserted (4) into the slabline (1) and create adjustable reflection factors when approached to the central conductor (2) of said slabline (1). Capacitive coupling between said probes (slugs) and central conductor is the main cause for reflection of the electromagnetic waves travelling across the slabline. Said probes (3) are also movable in horizontal direction (5), parallel to the central conductor (2) of said slabline and allow changing also the phase of said reflection factor at RF frequencies. The area (6) on a Smith chart (7) which can be reached by the reflection factor of said impedance tuners (FIG. 2) is called tuning range. The Smith chart is a polar representation of electrical impedances Z=R+jX, using a reflection factor Γ=(Z−Zo)/(Z+Zo), where Zo is the characteristic impedance of the transmission media, in our case 50 Ohms. The tuning range is important if transistors with very low or very high internal impedance need to be tested. The characteristic impedance of said Smith chart being 50 Ohms a tuner capable of creating reflection factors of 0.9 can reach impedances between a minimum of 2.6 Ohms and a maximum of 950 Ohms.
Impedance tuners with two, three or more similar or identical probes (FIGS. 3, 4 and 5) moving independently inside the same airline [1, 2] are used for multiple purposes, such as high reflection tuning [3] or harmonic tuning [4]. In order to achieve the harmonic or pre-matching (high reflection) tuning behavior, the airlines of said tuners must have a minimum length, allowing the carriages of the probes to travel freely horizontally. Each carriage must travel a minimum of one half of a wavelength in order to generate a phase control of 360° in reflection. For a tuner with N carriages this minimum length is N×Lambda/2, where Lambda is the wavelength at the minimum frequency of operation and N is the number of probes; i.e. a two probe tuner must be at least Lambda long and a three probe tuner 1.5×Lambda long. The wavelength being approximately:Lambda [mm]=300/Frequency [GHz]  {1}
For instance, at F=1 GHz the wavelength is Lambda=300 mm (11.8″). A two probe tuner would then be at least 11.8″ long and a three probe tuner 17.7″ long. At lower frequencies the length is proportional to the inverse of the frequency ratio. The real length of the tuner apparatus is larger than that, because the size of the mobile carriages, the external walls and the connectors of the airline are also to be accounted for. Typical sizes of carriages are 1.5″ to 2″ wide, the walls 0.5″ minimum and the connectors another 1″ each. So the three probe tuner (FIG. 4) operating at a minimum frequency of 1 GHz will, in reality be 17.7″+3×2″+2×0.5″+2×1″=26.7″ (≈290 mm) long. Typical applications require harmonic tuners starting at 800 MHz or 400 MHz. Such tuners are 31.15″ long at 800 MHz and 53.3″ (≈1,350 mm) long at 400 MHz (FIGS. 3, 4). The size and associate weight of said tuners makes it very difficult firstly to manufacture them with enough precision and secondly to carry and mount them on wafer probe stations, in order to test transistor chips with extreme positioning precision and resolution.
Therefore a new solution for the size problem is needed. The only known tuner configuration where an ordinary slabline is used to handle two probes sharing the same length of slabline, albeit only of high reflection applications can be found in [6 and 8]. The present invention describes a different configuration, where the two, three or four probes share the same length of airline (FIGS. 6, 7), but said airline comprises three or four slots instead of two known from prior art. Calibration and tuning algorithms are already existing and used in traditional harmonic tuners already on the market [2].